The hull of a ship, in particular a sailboat, is often provided with a keel. A keel imparts a greater stability to a ship in particular, and in the case of sailboats, an uprighting torque is created against the wind force acting on the sail and lateral drift is reduced. Since the keel increases the draft of a ship and thus the ship can run onto ground in a shallower body of water, for example, in ports and in near-shore areas, retractable keels have already been developed for larger sailboats, so that the keel is extracted from the hull by means of a driven lift device and can be retracted into the hull at a lesser depth of water.
The construction and practical manufacturing of a movable keel have proven to be extremely complex and difficult however because of some general boundary conditions in shipbuilding. The keel ensures the stability of a ship and should therefore typically constitute 30-40% of the total weight of a ship, thus necessitating a suitably dimensioned lift device for the retraction or extraction of the keel as well as a stable receptacle for anchoring and locking the keel to the hull of the ship. Because of the enormous forces prevailing in and on the keel, in particular as a counterweight on sailboats, the keel itself must be constructed and manufactured with a high level of stability. Unanticipated collision or contact with obstacles (e.g., rocks) in the water has been considered to be one of the most difficult scenarios to control. Such a so-called crash must be absorbed without any major destructive effect.
In addition, the securing and/or locking of the keel in the receptacle in the extracted position must be smooth running at all times and must also function even after a crash. This is not the case with the known retractable keel, because the keel is secured by the guidance of the lift device, e.g., by a hydraulic lift cylinder and/or one or more locking bolts arranged horizontally which cannot usually withstand the loads in a crash and are destroyed. To counteract this, the lift devices are usually over-dimensioned in relative to the actual lift function, which has a negative effect on costs and installation height.
Another disadvantage of this previously known lifting and securing arrangement with horizontal locking bolts is already apparent from the normal use in normal seas because the guidance of the lift devices and/or the horizontal locking bolts must be knocked out and replaced quickly. However it is practically impossible to accurately manufacture locking bolts which prevent removal from the beginning while at the same time permitting easy locking even when the keel has not been extracted in a precisely accurate manner.
Another disadvantage of the known retractable keels may be regarded as the fact that they require an excessive dimensioning in relation to the size of the hull because of the requirements described above and they take up a great deal of space in the interior of the ship so that it is often difficult for people to pass by the hull.
Such driven retractable keels can be found today only on special designs because the requirements and problems mentioned above make it difficult to have a simple enough cost-efficient production of such movable keels. The high construction and manufacturing costs prevent large scale use in shipbuilding.